U.S. Pat. No. 4,828,068 discloses a hydraulic rack and pinion steering assembly for steering a pair of steerable vehicle wheels. The assembly includes an elongate rack supported for longitudinal movement. The rack has opposite ends connected with the pair of steerable vehicle wheels, and has rack teeth in meshing engagement with a pinion. A hydraulic motor moves the rack longitudinally to effect steering movement of the vehicle wheels. The hydraulic motor has a pair of stationary end walls fixed to the interior of a housing through which the rack extends, and has a movable piston fixed to the rack for movement between the stationary end walls. When the rack is in a centered position, the rack teeth are spaced axially from the adjacent stationary end wall of the hydraulic motor so that the rack teeth remain outside the hydraulic motor when the rack moves. The rack thus has a total length defined in part by a portion of the rack that extends axially between the rack teeth and the hydraulic motor when the rack is in a centered position.
U.S. Pat. No. 3,951,045 discloses a hydraulic rack and pinion steering assembly including a rack which is shorter, in relative terms, than the rack disclosed in the '068 patent. The rack disclosed in the '045 patent moves longitudinally in opposite directions relative to a housing, and extends through a rack support tube which is fixed to the housing. A piston tube is movable telescopically between the rack support tube and an outer cylindrical tube, and is connected to the rack for movement with the rack. A piston is fixed to the piston tube for movement with the piston tube between the rack support tube and the outer cylindrical tube. A first variable volume hydraulic chamber is defined between the cylindrical tube and the piston tube on one side of the piston, and a second variable volume hydraulic chamber is defined between the cylindrical tube and the rack support tube on the other side of the piston. When the rack is in a centered position, the rack teeth within the rack support tube overlap axially with the adjacent hydraulic chamber. The rack does not have a portion that extends axially between the rack teeth and the hydraulic motor, and thus is shorter, in relative terms, than the rack disclosed in the '068 patent.
The structure of the steering assembly disclosed in the '045 patent enables the rack to be relatively shorter, but causes an imbalance in the hydraulic system. The tubular wall of the piston tube occupies space radially between the rack support tube and the cylindrical tube. The first hydraulic chamber is therefore narrower in the radial direction than the second hydraulic chamber. The surface area of the piston facing the first hydraulic chamber is consequently less than the surface area of the piston facing the second hydraulic chamber. The unequal surface areas of the piston facing the first and second hydraulic chambers are the effective piston surface areas against which hydraulic fluid pressure acts to move the piston. The hydraulic system is inherently imbalanced because the effective piston surface areas are inherently unequal.